OUT-OF-PLANE PHASE SEGREGATION AND INPLANE CLUSTERING IN A BINARY MIXTURE OF AMPHIPHILES AT THE AIR-WATER-INTERFACE

We have carried out extensive Monte Carlo simulations of a microscopic lattice model of a binary mixture of amphiphilic molecules, of two di fferent lengths, in a system where water is separated from the air abo ve it by a sharp well-defined interface. We have demonstrated an entro py-driven phase segregation in a direction perpendicular to the air-wa ter interface when the initial total surface density of the amphiphile s is sufficiently high. We have also investigated (a) the conformation s of the amphiphiles, (b) the distribution of the sizes of the cluster s of monomers belonging to the long amphiphiles as well as of those be longing to the short amphiphiles in planes parallel to the interface, (c) the effects of varying the lengths, total concentration, and the r atio of the numbers of the two types of the amphiphilic molecules as w ell as those of varying the strength of the intermonomer interactions and temperature. We have critically examined the interpretations of so me recent experimental results in the light of our observations.